When a motor vehicle equipped with a fuel-injected internal combustion engine and an automatic transmission is range-shifted from Park or Neutral (P/N) to Drive or Reverse (D/R) during engine idle, the sudden load on the engine output shaft produces a temporary error in the engine air/fuel (A/F) ratio. This error, in turn, may cause a significant engine speed (RPM) sag or even a stall.
Speed/density fuel injection systems, regulated by an onboard vehicle Engine Control Module (ECM), rely on intake manifold absolute pressure (MAP) and crankshaft rotational speed measurements to calculate the required amount of fuel to inject. FIG. 1 illustrates a typical cylinder intake valve timing event chart for a double-fired fuel-injected engine rotating at a typical idle speed of 800 RPM.
The chart is broken up into intervals of 12.5 msec, which corresponds to the loop time of the ECM fueling program. For convenience, the timing chart interval labeling begins at 0 msec. Each reference pulse corresponds to 180 degrees of crankshaft rotation. MAP is measured at the trailing edge of each reference pulse, while fuel is injected at the trailing edge of alternate reference pulses. The MAP measurement used for fuel calculation for the intake valve event shown occurs at 0 msec. The MAP measurements at 37.5 and 75 msec (not shown) occur too late to be used for fuel calculation for the intake valve event shown at 50 msec.
The ECM performs fuel calculations every 12.5 msec, regardless of engine RPM. Fuel is injected at 37.5 msec based upon the most recent fuel calculation, which took place at 25 msec. Fuel calculations made at 7.5 msec and beyond (not shown) occur too late to be used for fuel injection for the intake valve event shown.
The intake valve event begins at 50 msec and is approximately 50 msec in duration. Once it has ended, no more fuel can be added to that particular cylinder for the upcoming combustion event. An intake valve event for a particular cylinder occurs every 150 msec in a four cylinder engine at 800 RPM.
As can be seen from FIG. 1, 100 msec elapse from the time of the MAP measurement used for fuel calculation to the end of the intake valve event. Any increase in MAP during this time will force additional air into the cylinder without being compensated with the proper amount of fuel, producing an incorrect A/F ratio. This problem is the result of controlling an event-based device (internal combustion engine) with a time-based control system (ECM).
A P/N to D/R range shift applies an additional load to the engine and causes a rapid increase in MAP, requiring additional fuel to maintain the proper A/F ratio for preventing any disruption of engine RPM. Time-based measurements are unable to accurately detect rapid increases in MAP. A/F ratios based upon MAP measurements taken during range shifting will result in an insufficient amount of injected fuel, producing a lean A/F ratio which will further result in RPM sag or an engine stall. This effect is pronounced in control strategies which attempt to run the engine as lean as possible prior to catalytic converter warm-up.